Note* I've uploaded a new version (v6) with ribs that prevent the bottom from distorting.
This is based upon my previous duct - which is based upon MasterFX's.
Changes are; a lager slot for more airflow, bottom more horizontal, and overall nicer look.
The screw holes are intentionally covered with a very thin layer. A screw should easily pierce through. You should only need two screws so I didn't see a need to leave the other holes open.
No supports required for printing.
This duct clips securely onto the extruder assembly using no fasteners. The single fan provides for a nice curtain of air flowing across the print. Adding another fan or a larger fan would serve no useful purpose because there is still a significant restriction, the area of the slot is smaller than the area of the fan. These "muffin" fans are designed to move a volume of air, not build up pressure. The much heavier blower type fan is better for building pressure.
There are many very cool fan ducts here on Thingiverse. The ones you see that use muffin fans and have tiny openings for the air to come out, while they may work fine, are extremely inefficient. This design is intended to make better use of the muffin fan's abilities.
Redrawn using Autodesk Fusion 360